Halogens are a group of non-metallic elements that includes Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I) and Astatine (At). The elemental form of halogens are not found in nature. These generally combine with other elements or one another to form halides. For example, two chlorine atoms combine together to form one chloride molecule.
Upon mixing with other halides, Fluorine is found to be the most stable halogen because the stability of halide ions decreases down the group.